Currently free space optical communication is realized using a point source as a transmitter and a single detector receiver. A single detector receiver has a basic operational limitation that makes its use more difficult in the field, in that the field of view is very small, and thus mandates accurate alignment of the receiver with the transmitter. A large angular aperture, typically of at least 5°, would make use of such a system much simpler, since it would only be necessary for the person receiving the signal to know the approximate sector in which the transmitting party is situated, instead of the need to pinpoint their location more accurately. The requirement for the transmitter to accurately aim at the receiver depends upon the need to maintain covert transmission conditions.
A further requirement for the detector of such a free space optical communication receiver is that it should have the highest possible signal-to-noise ratio (SNR), which means that it should have as large an aperture as possible, and the it should have as low a noise level as possible. The latter requirement can be fulfilled by using a photomultiplier tube (PMT) detector, which is a large area detector having high sensitivity because of its internal amplification. However, the cost of a photomultiplier detector is high, and semiconductor detectors present a much more practical option. Such detectors are generally small area detectors, and include such detectors as the avalanche photodiode (APD) and the PIN photodiode. There are trade-offs between the use of these various types of detectors, as now discussed.
The main limitation of a large area, low noise detector such as a PTM, is the quantum noise caused by the background radiation. The use of a large numerical aperture leads to large amounts of collected background radiation and hence high quantum noise. Such quantum noise attenuates the SNR of the channel, and neutralizes the “low noise” advantage of a PMT.
As a numerical example, for a 50 mm. entrance aperture PMT with a 10° angular aperture and a quantum efficiency of 10%, at a bit rate of 4 MB/sec, the effective noise equivalent power (NEP) is 12.5 nW. Such a noise level is similar to the noise of a large PIN detector and is approximately 100 times greater than the noise of a low noise detector. In addition to the quantum noise, there are several additional disadvantages of the PMT, such as its high cost, the low quantum efficiency of a photocathode based detector, and the gradual degrading of the sensitivity of the detector with use.
On the other hand, a low dark noise semiconductor detector, such as an APD, will have an entrance aperture of less than 0.5 mm, and generally less than 0.2 mm, and in order to collect light for such a detector using a 30 mm diameter lens, the field of view (FOV) will be less than 0.5°. Such a small FOV for a free space optical communication application, would require precise alignment of the receiver with the transmitter, and is not therefore suited to tactical needs in the field. The main limitation to realizing a free space tactical optical communication system is the strong dependence between the sensitivity of the receiver and the need for precise alignment of the receiver towards the transmitter. Since a sensitive receiver can only be practically realized using small area detectors, but this leads to a small angular aperture, making use more problematic.
There therefore exists a need for an optical communication system incorporating a receiver which overcomes at least some of the disadvantages of prior art systems and methods, and in particular, which dispenses with the need for the accurate line of sight alignment required by such prior art systems.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.